A plasma is an electrically conductive gas containing charged particles. When atoms of a gas are excited to high energy levels, the atoms loose hold of some of their electrons and become ionized producing a plasma containing electrically charged particles - ions and electrons. It is well known as shown in the example below that this dissociation is key to processing powders.
The proposed invention is about a new device which produces an unique convective plasma in the 103 C temperature range (mid temperature plasma). The bulk of fundamental plasma research has mostly been concentrated on very high density, high temperature plasmas or cold room temperature plasmas. When such plasmas are used for materials processing the processing is also mostly carried out inside the plasma chamber whereas the proposed plasma device is a convective plasma generator capable of transfer along channels. Although very high temperature plasmas are now commonly available, almost all metallurgical processing work involves temperatures of the order of 103 C which falls in-between the two extremes where the bulk of plasma research has been carried out. Consequently, most of plasma devices have never been able to assist metal fabrication in an efficient manner except for coatings, and micro-bacterial cleaning/coatings where notable strides have been made. Some induction coupled plasmas (4000-12000 C) and transferred arc plasmas (xcx9c6000 C) which have been used in metallurgical industrial practices for gas modification or for the development of high temperature deposited coatings (e.g. plasma deposition of thermal barrier coatings on aircraft blades). These plasmas are difficult to direct or transfer in tubes to locations other than where they exist (typically between electrodes).
Several uses for very low temperature (cold) plasmas are also known for polymer systems processing. Cold plasmas are used for polymer surface cleaning or polymerization purposes. The effect of a plasma impingement on a given material is determined by the chemistry of the reactions between the surface and the reactive species present in the plasma. At the low-exposure energies typically present in glow-discharge plasma systems, the interactions occur only in the top few molecular layers. This layer is deeper for higher temperature plasmas. Plasma surfaces have unique reactions which are well known for low temperature plasmas and polymers but not as well know for metallic surfaces and medium temperature plasmas. In the case of polymers which are treated by cold plasmas, the gases, or mixtures of gases, used for the cold plasma treatment include air, nitrogen, argon, oxygen, nitrous oxide, helium, tetrafluoromethane, water vapor, carbon dioxide, methane and ammonia. Each gas produces a unique plasma composition and results in different polymer surface properties. For example, the high surface energies required for wettability and chemical reactivity may be increased very quickly and effectively by plasma induced oxidation, nitration, hydrolyzation, or amination (ammonia processing). Conversely, plasma induced fluorination depresses the surface energy, producing an inert and non-wettable surface. Such affects are often utilized for powder coating.
The two extreme plasmas (very hot and room temperature) mentioned above are mostly unsuitable for metallurgical work because of the extreme temperatures and very poor efficiencies. Although some plasma temperatures from conventional generators may be manipulated to have lower temperatures, there are other problems for economical use when such modifications are attempted. For example transferred arc induc ion plasmas are noisy and extremely costly for use in the 700 C range of temperatures where aluminum is melted and cast. Additionally, the conversion efficiency and power transfer efficiency of the transferred arc plasma is very low (single digits for these low temperatures) thus negating economical use. A new mid temperature range (700 C1300 C) convective plasma device is described herein. This new system is extremely quiet and seemingly offers the possibility of close to 100% power transfer efficiency. The use of this source with the novel heat transfer mechanism is expected to give rise to a host of new energy efficient technologies. It is to be noted here that until the availability of the this technology for medium temperature plasmas it was commonly recognized that thermal plasma processing faces a untenable economic prognosis in commercialization. Whenever conventional plasmas were considered in the past for metallurgical processing, invariably cost considerations prevented large scale applications, a fact highlighted often in the classic review by Pfender (E. Pfender, Plasma Chemistry and Plasma Processing, Vol.1, No.1, 1999, pg.1-28). The importance of this invention is made more obvious by the manageable cost of systems which can now become available. It is important therefore to develop processes with medium temperature plasmas. In this light, several plasma processes contemplated in (Plasma and laser Processing of Materials, eds. K. Upadhaya, TMS, 1991, and Carbide, Nitride and Boride Materials Synthesis and Processing, eds. Alan W. Weimer, Chapman Hall, 1997, could also benefit with the new source (this invention).
The plasma of this invention also may be used to vastly enhance heat transfer to a solid in order to improve productivity and save in the power lost to the surroundings by (i) concentrating the heat on the solid on account of the charge separation in the plasma and (ii) saving energy by processing faster such that the time in which it takes to melt a solid is so low that the surrounding device has little time to loose heat.
In an example below we find that heating the solid by the plasma of this invention uniquely allows the heat transfer coefficient to increase by orders of magnitude when compared to heating only by convection without plasma. Although, plasmas have been used to heat solids in the past, most heating configurations with the solids involve holding the solid (often a powder) inside the plasma. In the designs now possible, described below, the plasma is directed at the solid inside a chamber containing the surfaces to be heated which is kept far away from the plasma generating source which also provides for forced convection. Such solids can for example be aluminum ingots or scrap aluminum parts which require melting or iron surfaces requiring nitriding. The theoretical reason to expect such a benefit in heat transfer coefficient is discussed below.
Theoretical determination of the heat transfer coefficient (H) is an extremely difficult theoretical problem therefore numerous empirical and semi-empirical correlations are used to describe heat transfer to a spherical particle in a laminar flow. One of the popular methods is the Ranz-Marshall formula which describes the Nusselt number (Nu) for heat transfer to a spherical particle.
Nu=A+B RenPrm 
Here the constants A, B, n, m, are typically, 2, 0.6, 0.5; 0.5 respectively for forced convective flow over small particles.
Here Nu and Re are Nusselt and Reynolds numbers, respectively. These numbers are defined by the following equations:
Nu=Hdp/{overscore (xcex)}pxe2x80x83xe2x80x83(6) 
Re=[dp/vfxe2x88x92vp/{overscore (xcfx81)}f]/{overscore (xcexc)}f 
(see below for definition of symbols)
And where Pr is the Prandtl number defined as
Pr={overscore (c)}f {overscore (xcexc)}f {overscore (xcex)}pxe2x88x921 
where pp is the density, cp is the specific heat capacity at constant pressure, xcexp is the thermal conductivity, Tp is the absolute temperature, rp and dp are the radius and diameter of the particle, the dynamic viscosity is xcexcf, the velocity is v. f and p signify fluid and particle respectively. The bars signify averaged temperature values. The particle temperature is the function of time xcfx84 and its radial coordinate r.
There is a problem with fitting data if fluid plasma conditions exist and the classic paper of Young and Pfender (R. M. Young and E Pfender 1987 Nusselt Number Correlations To Small Spheres In Thermal Plasma Flow, Plasma Chemistry And Processing Vol.7 No.2 Pg211-226) showed early on that a plasma Nusselt number could be envisaged or defined which yields a higher heat transfer value that any modification of the RanzMarshall formula. It is believed that this plasma Nusselt number is very high although no real data exists to demonstrate this theoretical assertion.
Gasses like Nitrogen are able to dissociate at low temperatures.
Effectively the following reactions are occurring.
N2+E=2N Diatomic molecule of nitrogen+energy gives 2 free atoms of nitrogen
2N+E=2N++2exe2x88x922 free atoms of nitrogen+energy gives 2 nitrogen ions and 2 electrons
The new technique being proposed here is that this energy be transferred to a electrically conduction surface such as aluminum in order to deposit the energy to the electrical surface without it being lost to the surroundings, thus making the energy transfer extremely efficient compared to ordinary convective heating.
Now on the surface of a part especially if the surface is electrically conducting, where electrons are available in abundance, the following reaction can occur:
2N++2exe2x88x92=2N+E 
2N=N2+E 
This is the manner in which heat automatically deposits on the surface of alumirum thus increasing the energy transfer rate (i.e. Hplasma) substantially. For powder applications nitrogen is a general purpose primary gas used alone or with hydrogen secondary gas. Nitrogen also benefits from being the cheapest plasma gas. Nitrogen tends to be inert to most spray material except for materials like titanium. Argon is probably the most favored primary plasma gas and is usually used with a secondary plasma gas (hydrogen, helium and nitrogen) to increase its energy. Argon is the easiest of these gases to form a plasma and tends to be less aggressive towards electrode and nozzle hardware in powder melting and deposition hardware. Most plasmas are started up using pure argon. Argon is a noble gas and is completely inert to all spray materials. Hydrogen is mainly used as a secondary gas, it dramatically effects heat transfer properties and acts as anti-oxidant. Small amounts of hydrogen added to the other plasma gases dramatically alters the plasma characteristics and energy levels and is thus used as one control for setting plasma voltage and energy. Helium is mainly used as a secondary gas with argon. Helium is a noble gas and is completely inert to all spray materials and is used when hydrogen or nitrogen secondary gases have deleterious effects. Helium imparts good heat transfer properties and gives high sensitivity for control of plasma energy. It is commonly used for high velocity plasma spraying of high quality carbide coatings where process conditions are critical.
To date no device exists by which plasma energy can be transferred in the mid temperature rage economically to a surface. This is the device and technique claimed in this application.
Today, hot air blowers based on US5,963,709 (incorporated herein fully) are used for a variety of applications including direct heating of part surfaces, incineration of gas particulates, and heating enclosed chambers. More particularly, hot air blowers can be used for refractory curing, plastics sealing, cleaning diesel exhaust, and retrofitting gas fired ovens and furnaces.
Such blowers typically comprise a blower fan, an electric heating element, and a housing of the heating element. The blower fan forces air into the housing through an inlet at one end of the blower. The air is then heated by convection and radiation as it passes near the heating element and is provided at the outlet at the opposite end of the blower.
Accordingly, it is desirable to construct a hot air blower that can produce higher gas temperatures than current hot air blowers. Furthermore, it is desirable to produce a hot gas blower that has higher energy efficiency than current blowers. Further more it is very important to produce hot gas blowers which produce and transfer plasma instead of simply hot un-dissociated hot gas because such a method dramatically improves the heat transfer coefficient. Moreover, it is desirable to produce a hot plasma blower that does not cause the metallic heating element used within it to crack when the element reaches a certain temperature relative to the air passing near it.
This is the mid temperature plasma blower described herein.
It is an object of this invention to provide a device and method for heating a gaseous flow that can impart plasma to the flow.
It is a further object of the invention that this plasma posses kinetic energy so that it may be able to be delivered convectively to a conductive or non conductive part surface. It is another object of the present invention to provide a device and method for heating a gaseous flow that has high energy transfer efficiency.
It is yet another object of this invention to provide a device and method for heating a gaseous flow that can be used with a metallic or ceramic (such as molybdenum disilicide, silicon carbide, zirconia, carbon or boron nitride) heating elements at high temperatures without causing the element to crack.
A further object of this invention is to provide a device and method for heating a gaseous flow that provides an ideal residence time for the flow.
Another object of this invention is to provide a device and method for heating a gaseous flow that utilizes a pair of porous materials to provide a tortuous path for the flow and an increased residence time for heating the flow. U.S. Pat. No. 5,558,760 (the ""760 patent) and U.S. Pat. No. 5,279,537 are incorporated in their entirety herein as it relates to the composition of the porous material. Note that the heating element described in the ""760 patent may be the porous material itself.
To achieve the foregoing and other objects and in accordance with the purposes of the present invention as described above, a device for heating a gaseous flow is provided having a first materials, a second materials, and a heat source. The first materials has an inlet side for receiving the gaseous flow, an inner side for discharging the gaseous flow, and a plurality of openings, the openings providing at least one passageway for the inlet side to the inner side. The first materials preferably comprise porous ceramic materials.
The second materials has an inner side for receiving the gaseous flow, an outlet side for discharging the gaseous flow, and a plurality of openings, the openings providing at least one passageway from the inner side to the outlet side. The inner side of the first materials and the inner side of the second material define a gap for providing residence time for gases passing therethrough. Preferably, the second material comprises a porous ceramic materials. It is also preferred that the ratio of the volume of the materials to the volume of the gap is 3. The heat source is in direct or indirect contact with the gaseous flow and provides heat thereto. Preferably, the heat source is an electric heating element.
A method of heating a gaseous flow is also provided comprising the steps of providing a first materials, a second materials, and a gap therebetween, and forcing a gaseous flow through the first materials, the gap, and the second material. The first material and the second material are preferably comprise a porous ceramic material. It is also preferred that the ratio of the volume of the materials to the volume of the gap is about 3.